1. Field of the Invention
The present invention relates to a serial type printing apparatus that progressively forms an image on a print medium by repetitively alternating a printing scan, that causes a print head to execute printing as it scans over the print medium, and a print medium conveying operation.
2. Description of the Related Art
The serial type printing apparatus generally employs a multipass printing method in order to reduce seam lines appearing at boundaries between printing scans and a density (or grayscale) unevenness caused by ink ejection characteristic variations among individual nozzles. Such a multipass printing is known to be able to produce an image whose uniformity improves as the number of passes (the number of printing scans) required to print a unit area is increased. The increased number of passes, however, is also known to cause a fall in throughput. Therefore, there is a growing call for a printing method capable of producing a high quality image with as small a number of passes as possible.
Possible methods for improving throughput include adopting a bidirectional multipass printing. In the bidirectional multipass printing, a printing operation is performed in both forward and backward movements of a carriage on which a print head is mounted. When compared with a unidirectional multipass printing of the same pass number, the bidirectional multipass printing can reduce the number of its scans to about half that of the unidirectional printing, thus significantly reducing the time taken by the printing operation. It should be noted, however, that the bidirectional multipass printing has been known to have a problem of band-to-band variations in which density, color or glossiness of a printed image varies in a pitch that matches the print medium conveying distance. In this patent document, such variations among bands caused by the bidirectional multipass printing is referred to as “interband variations”.
FIGS. 1A and 1B explain in detail a cause and a phenomenon of the interband variations. FIG. 1A shows how a 4-pass bidirectional printing is done using a print head 1000 that has four nozzle arrays for ejecting cyan ink (C), magenta ink (M), yellow ink (Y) and black ink (K) respectively. The print head 1000 alternately performs a forward direction printing scan and a backward direction printing scan in an X direction (main scan direction) and, after each printing scan, moves in a Y direction (subscan direction) relative to the print medium by a distance equal to the width of a unit area.
Here let us focus our attention on the left end part of a first band having a width of a unit area. The area of interest is applied inks in the order of C→M→Y→K in the first printing scan and, after a relatively long period of time corresponding to almost one complete forward and backward scan of the print head, is applied inks in the order of K→Y→M→C. A left end part of a second band adjacent to the first band is applied inks in the order of K→Y→M→C and, after a relatively short period of time corresponding to almost only a reverse operation of the print head, is applied inks in the order of C→M→Y→K. In subsequent printing scans, odd-numbered bands including and following a third band that adjoins the second band are applied inks in the same manner as the first band; and even-numbered bands including and following a fourth band that adjoins the third band are applied inks in the same way as the second band. That is, two bands, that differ in the order of color ink application to the print medium and in the interval between ink application timings, are alternately arranged in the Y direction. The color ink application order and the ink application interval have some effects on density level, color hue or glossiness of an image formed on the print medium. As a result, in an image with its unit area bands, that differ in the ink application order and ink application timing, alternately arranged as shown in FIG. 1B, “interband variations” are observed.
To reduce a variety of image impairments including such “interband variations” observed in a multipass printing, it is effective to appropriately adjust a print permission ratio of individual nozzles arrayed in the print head. For example, Japanese Patent Laid-Open Nos. 2000-108322 and 2002-96455 describe a method that uses a mask pattern designed to give a predetermined deviation or bias to the print permission ratio of individual nozzles, thereby making “seam lines” and density unevenness less noticeable even if a small number of passes are used.
In maintaining the uniformity of an image it is effective, not only in the above patent documents but also in other forms of multipass printing, to have a means for controlling the print permission ratio in each printing scan of the multipass printing. This is because the degree to which the “interband variations” show up depends on various conditions such as the kind of print medium and the kind of inks used and, if the print permission ratio can be adjusted in each printing scan according to these conditions, the uniformity of the image can be maintained.
However, when the above print permission ratio is adjusted to suppress the “interband variations”, another image impairment problem may arise, such as “seam lines” between different bands and variations within each band. If, for example, the print permission ratio is differentiated among different printing scans over unit areas on a print medium, a plurality of nozzles arrayed in the print head are required to be divided into blocks of a width equal to the width of the unit area and the individual nozzle blocks be set to have different print permission ratios from one another. In that case, however, if there is a large difference in print permission ratio between adjoining nozzle blocks, a seam line may show up in narrow areas on a print medium printed by a boundary part of these nozzle blocks.
To deal with this problem, as disclosed in Japanese Patent Laid-Open No. 2002-96455, the seam lines can be made unnoticeable by progressively changing the print permission ratios of individual nozzles in each block so that a change in print permission ratio at a boundary between the adjoining nozzle blocks is not extremely large. However, where the print permission ratios of individual nozzles in each nozzle block are not made substantively constant or equal, variations or inconsistencies such as density variation, color unevenness or gloss unevenness may occur. In this patent document, a variation within a unit area that is produced by not making substantively constant the print permission ratios of individual nozzles in one nozzle block will be referred to as an “intraband variation”.
That is, the conventional bidirectional multipass printing has not been able to solve the problems of “interband variations”, “seam lines” and “intraband variations” at one time.